The field of LED lighting has made tremendous progress, however thermal management remains a challenge, in particular where the high light output is required relative to the allowable size of LED lighting system. This is a particular issue, namely, to replace high light output and high light density lamps and fixture. One such area that remains a challenge is high intensity discharge (HID) Lamps that have very high light output. For example, a 400 W HID lamp of less than 300 mm long and 120 mm diameter will have more than 35,000 lumens.
Types of HID lamps include metal halide lamp, high pressure sodium lamp and low pressure sodium lamps. These types of lamps require a warm up period of 1 to 15 minutes to reach 90% of their full light output. After a lamp has been operating for a period of time and then extinguished, it cannot be immediately turned back on. Before the lamp can be turned back on, the arc tube must have a chance to cool down or the lamp will not restart. This period of time is called the restrike time. Restrike times for traditional probe-start MH lamps can take 15 minutes or longer while restrike times for pulse-start MH lamps are generally much shorter. The long warm up time and long restrike time are a disadvantage. HID lamps may also contain mercury, a hazardous material, and may have only moderate life spans of about 10,000-20,000 hours, some may have rapid lumen depreciation in the first 3000-5000 hours.
In addition, HID lamps are an omni-directional light source which may be difficult to efficiently redirect into a more useful and efficient distribution. The optics used for redirecting the light can be expensive and lossy. For example, a fixture light loss factor, or the optical loss may typically be around 30%. So the moderate efficiency of a HID lamp is immediately discounted by 30%. It is also typically difficult to control the light, resulting glare that is not only wasted but a source of visual discomfort.